1. Field of the Invention
The present invention relates to heat exchange units which are adapted to have refrigerant flowing internally within a tube and simultaneously having the fluid to be cooled flowing externally over the same tube. More specifically, the present invention relates to high performance direct expansion coolers of the shell and tube type.
2. Description of the Prior Art
Heat exchangers of the shell and tube type have been commonly used for large commercial air conditioning and refrigeration applications wherein a circulating fluid typically water is cooled in the heat exchanger and thereafter circulated within the building to those specific areas where cooling is required. Often a shell and tube type heat exchanger is sold as a component of a packaged refrigeration unit having a conventional vapor compression refrigeration cycle. Therein refrigerant passes through a compressor where its temperature and pressure are increased and then proceeds to a condenser, where the refrigerant is cooled. From the condenser, the refrigerant flows through an expansion control device wherein the pressure of the refrigerant is decreased and finally the refrigerant flows to the shell and tube type heat exchanger wherein the liquid refrigerant changes state to a gaseous refrigerant absorbing heat from the liquid to be cooled in the process. Thereafter the gaseous refrigerant returns to the compressor where it is again compressed to commence the next cycle. Heat exchangers of the shell and tube type have also been sold separately within the refrigeration industry primarily as water chilling units in refrigeration machinery for commercial and business installations.
The typical direct expansion chiller or cooler has a multiplicity of parallel refrigerant carrying tubes mounted between headers communicating with inlet and outlet conduits within a cylindrical casing. The refrigerant is circulated through the tubes while the fluid to be cooled is circulated over the tubes. The refrigerant changes state within the tubes of the heat exchanger as it absorbs heat from the fluid to be cooled. The now cooled fluid may be circulated to meet the necessary cooling requirements of the installation. Previous heat exchangers have utilized copper or other material tubing with a smooth inner and outer surface more particularly referred to as prime surface tubes. Star shaped inserts have been available to create internal fins within the tubes, however, these have proved costly and have not been overwhelmingly accepted by the industry.
Tubes having integral helical internal fins have been known for sometime and are the subject of the following patents all by French, U.S. Pat. Nos. 3,422,518; 3,622,403; 3,622,582; 3,750,709; and 3,776,018. Other U.S. patents pertaining to metal tubes having internal fins include Laine; U.S. Pat. Nos. 511,900; Rieger, 3,768,291; Luca, 3,580,026; Issott; 3,118,328; Hill, 3,292,408; Koch, et al; 3,298,451; Nakamura, et al 3,830,087; Davis, 1,465,073; Lampart, 1,985,833; Diescher, 1,989,507; Hackett, 2,392,797; and Garand, 2,397,544.
Internal fin tubes have been commercially available for many years. Previous testing of these tubes in a typical shell and tube type exchanger has shown only minor improvement in overall unit efficiency. This prior testing was accomplished by substituting an internal fin tube for the existing smooth surface tube. It has now been discovered that efficient use of an internal finned tube requires a lesser temperature drop over the length of the tube circuit than the temperature drop over a standard smooth tube circuit. Furthermore, the internal finned tube shows negligible, if any, overall performance improvement when operated at the same temperature drop over the tube circuit as that of a smooth tube. Consequently to obtain the high efficiency desired from an internal finned tube it is necessary to select internal circuiting within the heat exchanger so that the temperature drop across the circuit is considerably less than across a similar circuit having smooth tubes.
It has further been found that the prior art internally finned tubes may be limited to a lead angle, that angle the fin makes with the axis of the tube, of approximately 15.degree.. It has also been found that tube performance is enhanced if this angle is increased, in fact, the tube performance is maximized at angles considerably larger than 20.degree..
In order to make effective use of internal fin tubes it has been found necessary to both increase the lead angle of the internal fin and to operate the heat exchanger with a temperature drop over the refrigerant circuit that is much less than previously utilized. Observing the above conditions, it is possible with internal fin tubing to substantially increase the capacity of existing shell and tube type heat exchangers by changing the circuiting within the heat exchanger to result in the appropriate temperature drop and by changing the lead angle within the tubes to maximize their heat exchange efficiency. This increase in performance is accomplished with very little cost increase and with very little additional assembly time required.